Two projects are proposed to investigate two methodological issues related to the use of Waksberg's procedures for random digit dialing in population-based case-control studies. The specific aims are: 1) to identify and optimal time when previously dialed ineligible telephone telephone numbers (defined as non- working, business, wrong county, fast busy, slow busy, and no contact telephone numbers can be returned to the sampling pool and 2) to determine if geographical clustering occurs more than would be expected by chance when more than one control is selected per Primary Sampling Unit (PSU). The general aims of both projects are: to develop guidelines and recommendations, 1) for the restoration of ineligible telephone numbers to the sampling pool, and 2) for the possible selection of more than one control per PSU. In Project 1, ineligible telephone numbers from six case-control studies dating back to 1979 will be identified. The percentage of ineligible telephone numbers (about 9,061 numbers) that have converted to residential numbers will be determined by calling the number and/or checking its status against the telephone company's ScanTel Directory Assistance Data Base. A household census will be taken at all residential numbers. These data will be used 1) to estimate changes in the probability of selection by age and sex that would occur if the numbers classified as ineligible were converted to residential numbers at various points in time and 2) to provide recommendations about the optimal time to return numbers to the sampling pool. In Project 2, the addresses of approximately 1,029 controls selected from three previous case-control studies will be mapped to determine if controls selected using different variations of the Waksberg method (e.g., primary numbers only, primary and secondary numbers) cluster more than would be expected by chance. The findings from both projects could have a significant impact on study design and study costs -- two areas that will be addressed in the recommendations that are made.